FIG. 1 depicts a conventional gas turbine engine power plant 20 of the ducted fan type. The major components of this conventional power plant include a gas turbine engine 22 for driving a fan 24 which rotates about the longitudinal axis 26 of the power plant and a bypass duct 28 which surrounds fan 24. Air is pumped through bypass duct 28 by fan 24 to increase the mass rate of flow through the power plant and thereby increase the thrust it generates. Power plants of this type are designed for aircraft applications.
During takeoff and flight, power plant 20 generates thrust acting in the direction opposite that indicated by arrow V.sub.oo in FIG. 1 (forward thrust). When the aircraft lands, the direction of the thrust and the direction of the mass flow through the fan, suggested by the arrow V.sub.FAN, may be reversed to brake the aircraft. This is accomplished by reversing the pitch of fan blades 30. With this done, fan 24 induces air into bypass duct 28 as suggested by arrow 32 and pumps the induced air out the front end 34 of the bypass duct.
In a conventional engine as shown in FIG. 1, air induced into bypass duct 28, in the reversed thrust mode of operating the power plant, tends to separate to a large degree from the inner surface 36 of the bypass duct (the extent of separation is indicated by reference character 37). This makes fan 24 relatively inefficient as only the roots of the fan blades 30 will then play any significant part in pumping the induced air through bypass duct 28.
It was thought that this problem of air separation during the reversed thrust mode of operation could be overcome by flaring the rear end of the bypass duct. A power plant embodying this innovation is illustrated in FIG. 2 and identified by reference character 38. As can be seen from FIG. 2, this solution to the problem of air separation was not viable. Despite flare 40, air induced into the rear end 42 of bypass duct 44 during reversed thrust operation of power plant 38 still separated from the inner surface 46 of the bypass duct to an unacceptable, performance degrading extent as indicated by shaded area 48.
Yet another scheme for solving the problem of detached air in the reversed thrust operation of a ducted fan power plant is employed in the power plant 50 identified by reference character 50 in FIG. 3 and described in detail in U.S. Pat. No. 3,820,719 issued June 28, 1974 to Clark for GAS TURBINE ENGINES. In that power plant, the fan bypass duct 52 has a stationary forward section 54 and a rear section 56 which can be rectilinearly displaced or translated away from the forward section in the direction indicated by arrow 58 during reversed thrust operation. This forms an annular secondary inlet 60 through which additional air can be induced into bypass duct 52.
The shaded area identified by reference character 62 in FIG. 3 shows that this approach to solving the air separation problem encountered in reversed thrust operation of ducted fan power plants is still not satisfactory. Furthermore, it is apparent from FIG. 3 that the air induced into bypass duct 52 through the secondary inlet 60 must turn and follow paths generally parallel to the common longitudinal centerline 26 of the power plant and the bypass duct. To the extent that the induced air does not follow such a path--one of which is identified by reference character 64 in FIG. 3--it will comingle with air induced in the bypass duct through the rear end 66 of the translatable bypass duct section 56. This causes degradation in the reversed thrust performance of the power plant.